This application claims Paris Convention priority of DE 199 22 652 filed May 18, 1999 the complete disclosure of which is hereby incorporated by reference.
The invention concerns a device for homogenizing the magnetic field generated in the working volume of a magnetic resonance device by a main magnet with a superconducting short-circuited main coil, wherein the working volume is surrounded by a plurality of ferromagnetic or permanent-magnetic elements whose number, strengths and positions are selected such that they, in their entirety, largely compensate field inhomogeneities of the main magnet in the working volume.
A device of this type is e.g. disclosed in EP 0 272 411 B1.
In contrast to magnetic field homogenization by means of so-called shim coils through which correction currents flow, so-called xe2x80x9cpassive shimmingxe2x80x9d has recently become more and more established, in particular for magnets used in magnetic resonance imaging (MRI). Actively controlled shim coils are replaced with passive ferromagnetic or permanent-magnetic small plates disposed about the working volume at predetermined locations. In standard superconducting tomography magnets having a room temperature bore, rod-shaped holders are inserted into axial guides on the wall of the room temperature bore, in which a predetermined number of ferromagnetic xe2x80x9cshim platesxe2x80x9d are stacked and fixed at predetermined axial positions.
Although e.g. superconducting magnets for high-resolution NMR still use shim coil sets for homogenizing the field at the sample location, magnetic resonance imaging (MRI) already uses the above mentioned method of passive shimming. This method is more demanding but less expensive and, importantly, more convenient, must be executed only once, and does not require additional current or power supplies.
A superconducting short-circuited magnet coil maintains a constant magnetic flux through its bore, i.e. the superconducting current changes immediately in response to e.g. an external disturbing field in such a manner that the total flux through the coil does not change. This, however, does not generally mean that the field in the working volume remains absolutely homogeneous and constant, since the spatial field distribution of a disturbance and of the main magnetic coil do not coincide. Prior art has proposed compensating these tolerances through arrangement of the main coil geometry, with additional superconducting coils, or using control measures (U.S. Pat. Nos. 4,974,113, 4,788,502, 5,278,503).
Integration of a passive shim system of the above mentioned kind poses a further problem. The magnetic field in the working volume has been shown to drift with time, i.e. the term of zero order of the magnetic field expansion does not remain constant at the center of the working volume. This effect is explained, in general, by temperature fluctuations in the room temperature bore of the superconducting magnet system and magnetization changes in the shim plates resulting therefrom. Magnetization of ferromagnetic iron plates is oriented by the main field magnet along its axis and saturated. However, saturation magnetization depends slightly on the temperature. This is also true for permanent-magnetic shims e.g. made from NdFeB, to an even greater degree.
Although such drifts can often be tolerated in conventional imaging, this is not the case for measurements which rely on the absolute frequency. In high-resolution spectroscopy, a so-called lock system is often used in conjunction with a compensation coil to compensate for drifting. However, for more and more applications, in particular in connection with switched gradients, the lock system cannot be used for spectroscopy.
For this reason, there is a need for a device of the above mentioned kind which is insensitive to temperature fluctuations in the shim elements.
This need is fulfilled in that the number, strengths and positions of the shims are calculated such that, in their entirety, they do not generate, to a good approximation, a magnetic field in the center of the working volume.
This boundary condition can be used to define the arrangement and optimization of the shim system: only optimization solutions are accepted which meet this condition (optionally, within narrow tolerances). For a temperature drift of the shims, the so-called B0 field thereby does not change and the resonance frequency in the center of the working volume remains constant (at the value without shim system) and does not drift. The shims generate the required inhomogeneous gradient fields in the working volume. In accordance with the invention, the superposition of all these gradient field contributions must be approximately zero at the center.
Within the scope of the invention, the main magnetic field can be generated in any manner, e.g. using permanent magnets, resistive or superconducting magnet coils having iron pole shoes, resistive or superconducting xe2x80x9cair coilsxe2x80x9d, or combinations thereof.
It is, however, particularly advantageous when the main field magnet comprises a superconducting short-circuited main coil. Such magnets have become largely standard in tomography systems and are used nearly exclusively in analytic NMR systems. They guarantee good homogeneity and above all stability with time, independent of the quality of the power supply.
In the present case, it is particularly advantageous to calculate the number, strengths and positions of the shims such that, to a good approximation, they, in their entirety, do not couple with the main coil. As mentioned above, a superconducting short-circuited coil xe2x80x9cresistsxe2x80x9d any flux change. Unfortunately, when a conventional shim system drifts due to temperature fluctuations, the flux through the main coil changes, and the superconducting current will generally change leading to a field change in the center of the working volume. This can be prevented if the total flux through the main coil generated by the shim system is zero. This boundary condition can also be required and satisfied (optionally, with close tolerance) for optimization of the shim arrangement, in particular, if some or all of the shim elements are permanent magnetic shim elements, which i.e. can be disposed such that their magnetization is opposite to the field at their locations.
The object can also be achieved with superconducting short-circuited main coils in that the number, strengths and positions of the shims are calculated such that, for a change in their overall magnetic moment, the resulting change in their total magnetic field in the center of the working volume is compensated, to a good approximation, by an opposing change in the magnetic field generated by the main coil due to inductive coupling to the main coil.
Elimination of individual disturbances is not necessary. It is sufficient to compensate for the sum of all disturbances.
Due to the additional degree of freedom, the use of permanent-magnetic shims generally provides better optimization compared to iron. This option is particularly advantageous in connection with the requirement of vanishing total flux through the main coil and justifies the higher price, the more complicated handling, and the somewhat larger temperature drifts.
Clearly, the permanent magnet material must be suitable for the given strength of the main field, i.e. must have sufficient coercive field strength. NdFeB is sufficient for most tomography systems (up to approximately 2 Tesla). With the higher field strengths of analytic magnets (currently up to approximately 20 Tesla) SmCo is recommended. Both of these materials can be used together, e.g. NdFeB for xe2x80x9cpositivexe2x80x9d contributions and SmCo for xe2x80x9cnegativexe2x80x9d contributions. The combination of iron (positive) and a permanent magnet (negative) can also be suitable.
With conventional tomography systems or superconducting magnets of analytic NMR having a cylinder-shaped room temperature bore, the shims are preferably disposed on a substantially cylindrical surface to guarantee free access to the working volume.
With pole shoe magnets or one-sided structures, the shims can preferably be disposed substantially in one or two planes.
Compensation or elimination of the field drift is most successful if all individual shims have the same temperature. This can be improved by connecting the shims in a heat-conducting manner.
Clearly, the features mentioned above and below can be utilized not only in the combination mentioned but also in other combinations or individually, without departing from the scope of the present invention.
The arrangements in accordance with the invention can be combined with or integrated in conventional shim systems. In particular, additional conventional shim coils can be provided.
The invention is explained in more detail below with reference to the drawings.